The present invention relates to plate-like magnetoplumbite type ferrite particles for magnetic recording and a process for producing the same. More particularly, the invention relates to the plate-like magnetoplumbite type Ba ferrite particles for magnetic recording having a large magnetization value and a proper coercive force and excellent in temperature stability specified by the change of coercive force at a temperature of -20.degree. to 120.degree. C. in the range of -0.5 to +0.5 Oe/.degree.C., and a process for producing such particles.
With the development of the art of magnetic recording in recent years, the request has grown for the magnetic materials for recording, especially ones for perpendicular magnetic recording comprising the ferromagnetic non-acicular particles having a proper coercive force and a large magnetization value as well as a suitable range of average particle size and excellent temperature stability, as for instance stated in Japanese Patent Application Laid Open (Kokai) No. 55-86103(1980).
Generally, magnetoplumbite type ferrite particles are known as ferromagnetic non-acicular particles.
As a technique for producing plate-like magnetoplumbite type ferrite, there has been known a method in which an alkaline suspension containing Ba or Sr ions and Fe (III) is subjected to a hydrothermal treatment by using an autoclave as reactor (this method being hereinafter referred to simply as autoclaving method).
The plate-like magnetoplumbite type ferrite particles for magnetic recording are required to have a proper particle size, specific magnetic properties such as a suitable coercive force and a large magnetization value, and excellent temperature stability and erasing characteristics.
Regarding, for instance, the particle size of the plate-like magnetoplumbite type ferrite particles for magnetic recording, such particles are required to have as small a diameter as possible, especially not greater than 0.3 .mu.m.
Such fact is mentioned in, for instance, Japanese Patent Application Laid Open (Kokai) No. 56-125219(1981) which states: "The advantageous utility of perpendicular magnetic recording for longitudinal magnetic recording becomes apparent in the recording wavelength region below 1 .mu.m. For conducting satisfactory recording and reproduction in such wavelength region, it is desirable that the crystal particle size of the ferrite particle is less than about 0.3 .mu.m. However, no desired ferromagnetism is provided when the particle size becomes smaller than 0.01 .mu.m, so that a crystal particle size in a range of 0.01 to 0.3 .mu.m is practically required."
As for the magnetic properties, the plate-like magnetoplumbite type ferrite particles for magnetic recording are required to have a coercive force usually on the order of 300 to 1,000 Oe. For reducing the coercive force of the plate-like magnetoplumbite type ferrite particles produced by the autoclaving method to a proper level, it has been proposed to substitute part of Fe (III) in the ferrite with Ti (IV) and Co (II) or with Co (II) and divalent metal ions M (II) such as Mn or Zn ions.
As to the magnetization value, it is required that such magnetization value is as large as possible. This is mentioned in, for instance, Japanese Patent Application Laid Open (Kokai) No. 56-149328(1981) which states: "Magnetoplumbite ferrite used as a material for magnetic recording media is required to have as large a degree of saturation magnetization as possible."
Also, the plate-like magnetoplumbite type ferrite particles, from FIG. 4 on page 1,123 of IEEE TRANSACTIONS ON MAGNETICS, MAG-18, No. 6, have a tendency to be increased in coercive force as the temperature rises, and are poor in magnetic (especially coercive force) stability to temperature (hereinafter referred to simply as temperature stability), so that the improvement of such temperature stability is required.
Further, the plate-like magnetoplumbite type ferrite particles have the problem of poor erasing characteristics as for instance described in Japanese Patent Application Laid Open (Kokai) No. 62-46430(1987) which states: "Magnetic media using magnetic powder of barium ferrite, although excellent in short wave recording properties, are poor in erasing characteristics, and especially when used for floppy discs, these magnetic media prove to be poor in overwrite (erasing rate) characteristics and have problems in use." Thus, the improvement of such erasing characteristics is also required.
The plate-like magnetoplumbite type ferrite particles having a large magnetization value and a proper coercive force as well as excellent temperature stability and erasing characteristics are most keenly required at present. In the autoclaving method described above, various kinds of magnetoplumbite type ferrite particles are precipitated according to the reaction conditions. These precipitated particles usually have a hexagonal plate-like shape, and their powder properties such as particle size distribution and average diameter and their magnetic properties such as coercive force, magnetization, temperature stability and erasing characteristics differ depending on the producing conditions.
For example, the plate-like magnetoplumbite type ferrite particles containing Co (II) and Ti (IV) obtained by producing the plate-like Ba ferrite particles containing Co (II) and Ti (IV), in which part of Fe (III) in the ferrite has been substituted with Co (II) and Ti (IV) for reducing the coercive force to a proper level, by using the autoclaving method, and heating and calcining the thus obtained particles are high in the effect of reducing coercive force by Co (II) and Ti (IV) and therefore can be controlled to a proper coercive force by the addition of a small quantity of the additives, so that the drop of magnetization due to the additives is minimized, allowing such particles to have a relatively large value of magnetization, for example, about 50 to 60 emu/g, but the temperature stability of these particles is poor, for example, the change of coercive force at a temperature of -20.degree.to 120.degree.C. in the range of +2.5 to 6.0 Oe/.degree. C. Thus, the plate-like magnetoplumbite type ferrite particles containing Co (II) and Ti (IV) have a tendency that their coercive force increase as the temperature rises. Such phenomenon can be also forecasted from FIG. 1 on page 1,459 of JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, No. 15-18 (1980).
Also, the plate-like magnetoplumbite type ferrite particles containing Ni (II) and Ti (IV) or Zn (II) and Ti (IV) obtained by producing the plate-like magnetoplumbite type ferrite particles containing Ni (II) and Ti (IV) or Zn (II) and Ti (IV), in which part of Fe (III) in the ferrite has been substituted with Ni (II) or Zn (II) and Ti (IV) for reducing the coercive force to a proper level, by using the autoclaving method and heating and calcining the particles are small in the effect of reducing coercive force by Ni (II) and Ti (IV) or Zn (II) and Ti (IV), so that it is necessary to use a large amount of additives for controlling the coercive force to a proper level, resulting in a large drop of magnetization value which becomes about 47 emu/g at highest. The temperature stability of these particles, as forecasted from FIG. 1 in the JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, is in the range of about +1.0 to +3.0 Oe/.degree.C. (in terms of possible change of coercive force at a temperature of -20.degree. to 120.degree. C.), which is better than that of the plate-like magnetoplumbite type ferrite particles containing Co (II) and Ti (IV) but is still unsatisfactory.
As means for improving temperature stability of the plate-like magnetoplumbite type ferrite particles containing the elements for reducing the coercive force such as Co (II) and Ti (IV), there are known, for example, the methods disclosed in Japanese Patent Application Laid Open (Kokai) Nos. 61-152003(1986) and 62-132732(1987).
According to the method of Japanese Patent Application Kokai No. 61-152003(1986), the plate-like magnetoplumbite type ferrite particles containing the elements for reducing the coercive force such as Co (II) and Ti (IV) are subjected to a heat-treatment at 300.degree.-700.degree. C. in a reducing atmosphere. But this heat-treatment enhances the coercive force more than twice that before the heat treatment, making it difficult to control the coercive force to a proper level.
According to the method of Japan Patent Application Kokai No. 62-132732(1987), the aspect magnetoplumbite type ferrite particles are produced so that their average diameter become less than 1.0 .mu.m, their thickness of the C-axis direction become less than 0.2 .mu.m and their plate-like ratio (average diameter of plate surface/thickness of the C-axis direction) become greater than 5. This method, therefore, has the defect that the improvement of temperature stability is subject to the limitations by the form of particles.
Also, the erasing characteristics of the conventional plate-like magnetoplumbite type ferrite particles containing the elements for reducing the coercive force such as Co (II) and Ti (IV) had a specific corelation with coercive force, that is, there was a tendency that the erasing characteristics be deteriorated as the coercive force increases, the erasing characteristic becoming about 40 dB when the coercive force is about 500 Oe, the erasing characteristic becoming about 30 dB when the coercive force is about 700 Oe, and the erasing characteristic becoming about 20 dB when the coercive force is about 900 Oe. A method for improving the erasing characteristics of the plate-like magnetoplumbite type ferrite particles is disclosed in the Japanese Patent Application Laid Open (Kokai) No. 62-46430(1987). According to this method, the plate-like magnetoplumbite type ferrite particles are produced so that their average diameter become less than 0.2 .mu.m, preferably less than 0.15 .mu.m, more preferably less than 0.1 .mu.m, and the aspect ratio (average diameter of the plate surface/thickness of the C-axis direction) become greater than 6, more preferably greater than 8, but this method has the defect that the improvement of erasing characteristics is subject to the limitations by the form of particles.
Thus, the establishment of a process capable of producing the plate-like Ba ferrite particles having a proper coercive force and a large magnetization and also improved in temperature stability and erasing characteristics without restrictions by the form of particles has been strongly required.
As a result of many studies for obtaining the plate-like magnetoplumbite type ferrite particles having a proper coercive force and a large magnetization value and also improved in temperature stability and erasing characteristics without restrictions by the form of particles, it has been found that the plate-like magnetoplumbite type ferrite particles containing a metal (II) and a metal (IV) and having zinc incorporated as a solid solution close to the particle surface, obtained by a process comprsing subjecting an alkaline iron hydroxide (III) suspension containing ions of one metal (II) selected from Ni and Zn, ions of one metal (IV) selected from Ti, Sn and Zr, and Ba or Sr ions to an autoclaving treatment at a temperature of from 150.degree. to 330.degree. C. to produce the plate-like magnetoplumbite type ferrite particles containing the metal (II) and metal (IV), suspending the particles in a zinc-containing aqueous solution of pH 4.0-12.0 to obtain the plate-like magnetoplumbite type ferrite particles containing the metal (II) and metal (IV) and having a hydroxide of zinc deposited on the particle surface, filtering and drying the particles, and then heating and calcining them at a temperature in the range of 600.degree.-900.degree. C., have a coercive force suited for magnetic recording and a large magnetization value and are also excellent in temperature stability and erasing characteristics. The present invention was attained on the basis of such finding.